In a photolithography process for manufacturing semiconductor devices, an exposure apparatus which projects and exposes a pattern image of a mask (or a reticle) onto a photosensitive substrate (e.g. wafer or glass plate on which photoresist is coated) via a projection optical system has been used. For the exposure apparatus, the resolution required for the projection optical system is increasing more and more as the degree of integration of semiconductor devices improves.
In order to satisfy the demand for the resolution of the projection optical system, the wavelength λ of the illumination light (exposure light) must be decreased, and the numerical aperture NA at the image side of the projection optical system must be increased. Specifically, the resolution of the projection optical system is given by k·λ/NA (k is a process coefficient). The numerical aperture NA at the image side is given by n·sin θ, where n is a refractive index of the medium (normally a gas, such as air) between the projection optical system and the photosensitive substrate, and θ is a maximum incident angle to the photosensitive substrate.
When it is attempted to increase the numerical aperture at the image side by increasing the maximum incident angle θ, the incident angle to the photosensitive substrate and the emission angle from the projection optical system increase, and reflection loss on the optical loss increases, so a large effective numerical aperture at the image side cannot be secured. Therefore an immersion technology for attempting an increase of the numerical aperture at the image side by filling a medium, such as liquid, having a high refractive index in an optical path between the projection optical system and the photosensitive substrate, is known (e.g. Patent Document 1).
Patent Document 1: International Publication No. WO2004/019128 Pamphlet